1. Field of the Invention
The present invention relates to a method of processing the end of a plastic pipe to form a pipe end socket, and a movable molding device suitable for carrying out this method. More particularly, the present invention relates to a method of molding PVC pipe joints, and a device therefor.
2. Description of the Prior Art
U.S. Pat. No. 3,557,278 discloses a method of molding the end portion of a PVC pipe to form a wall-thickened socket including arranging an outer frame around a socket molding core, making the space formed between the core and the outer frame substantially equal to the wall thickness of PVC pipe, and forcedly pressing the heated end of PVC pipe into the space.
In FIGS. 1A through 1D attached hereto for reference, (a') represents a slanted portion, (b') a linear portion and (c') a mountain or increased radius portion.
(P') denotes the heat-softened end of PVC pipe, which is pressed into the space between the core and the outer frame by pushing the same. As shown in FIG. 1A, the wall of the softened end of PVC pipe is thickened by compression force (f) acting in the axial direction of pipe when it reaches the slanted portion (a').
As the forced insertion portion of pipe advances, the compression force becomes larger and, therefore, the sectional area of the pipe also becomes larger. However, since the radius of the pipe is also enlarged, the wall thickness of the pipe portion whose radius has been enlarged is reduced and, as shown in FIG. 1B, a gap (g') is created at the linear portion (b') between the inner surface of outer frame and the outer surface of the pipe.
The thickness (d) of the gap (g') can be qualitatively understood as follows: Namely, assuming in FIG. 1B that the sectional area of PVC pipe at a point (P.sub.0) is (S) and that the inner radius of the pipe is (r), the outer radius (R) of pipe can be expressed as follows: EQU .pi.(R.sup.2 -r.sup.2)=S
Therefore, ##EQU1## Assuming that the distance of space between the core and the outer frame is (t), the thickness of gap can be expressed as follows: ##EQU2##
When the forced insertion of the PVC pipe end is further advanced and the foremost end of pipe comes to the first stepped portion (c.sub.1 ') of the portion of increased radius (c') as shown in FIG. 1C, the compression force (f) acting in the axial direction of pipe is further increased to thereby attain a high speed wall thickening effect, and the wall of the PVC pipe located at the linear portion is thickened by the compression force (f). However, according to the observation results conducted by the inventors of present invention and others, it has been found that, because the distance (d) of the gap (g') over the PVC pipe at the linear portion (b') is large, buckling occurs in the PVC pipe in the linear portion at the initial stage at which the wall of PVC pipe is thickened by the compression force and a crease is created on the outer surface of socket.
Further, Japanese Patent Publication No. 25871/52 discloses a processing method comprising a step of thickening the wall of the softened plastic pipe end between the core and the outer frame without changing the inner radius of pipe; forming a suitably cooled film on the inner and outer circumferential walls of wall-thickened pipe end; feeding the wall-thickened end of pipe onto the radius enlarging portion of core to thereby reduce the thickness of that end and enlarge the inner radius thereof, said core having no outer frame; and projecting from the inside a ring-shaped projection at the region of this radius enlarging portion and molding an inner ring-shaped groove same as the shape of the projection in the inner circumferential wall of wall-thickened and radius-enlarged pipe end. However, it is impossible in this processing method to increase to such an extent as desired the thickness of the pipe end on the radius enlarging portion.
The method of molding the plastic pipe end to a wall-thickened socket requires a high level of engineering. The apparatus employed to carry out the processing method of U.S. Pat. No. 3,557,278 includes the structure in which half of the core is supported by a horizontal guide, while the remaining half of the core is supported by a vertical guide perpendicular to the horizontal guide, thus allowing the core to be drawn outside after the socket is molded. There is also disclosed a supporting mechanism large enough to resist a large resistance generated between the pipe and the core, but the use of this large resistance is primarily neglected.